Ion trap mass spectrometers typically allow son scanning by essentially filling an ion trap in a mass-independent manner and emptying the trap in a mass-dependent manner by manipulating the RF and DC voltages applied to one or more of the electrodes. The ion storage and fast scanning capabilities of the ion trap are advantageous in analytical mass spectrometry. High analysis efficiency, compared to typical beam-type mass spectrometers, can be achieved if the time to eject and detect ions from the trap is smaller than the time required to fill a trap. If this condition is met, then very few ions are wasted.
Linear quadrupoles have been widely used in some mass spectrometers for many years. Generally, these devices are constructed from four parallel rods within which a two-dimensional quadrupole field is established (in the x-y plane). Mass selection is achieved by appropriately choosing a combination of radiofrequency (RF) and direct-current (DC) voltages, such that ions within a very narrow mass-to-charge window are stable over the length of the quadrupole.
Conventional ion trap mass spectrometers, on the other hand, operate with a three-dimensional quadrupole field. These instruments are capable of very high efficiencies, since the time to fill the ion trap and generate a complete mass spectrum can be very short. A problem with 3-D ion traps is that they generally have poor trapping efficiencies, such as less than 10%, for externally generated ions. This is primarily due to their small volume. This small volume also results in a limited dynamic range, since there is a maximum charge density beyond which the response of the top becomes nonlinear with respect to ion number, and the quality of the mass spectra deteriorates.
The advantages associated with the trapping of ions in linear traps can include the following. Linear ion traps have a very high acceptance, since there is generally no quadrupole field along the z-axis (axial/parallel to the rods). Ions admitted into a pressurized linear quadrupole undergo a series of momentum dissipating collisions with a carrier gas in a collision cell, effectively reducing axial energy prior to encountering the end electrodes, thereby enhancing trapping efficiency. That is, the reduced momentum avoids requiring a large DC barrier to contain ions in the axial direction. Larger volume of the pressurized linear ion trap relative to the 3-D device also means that more ions can be stored prior to the onset of any deleterious effects of space charge. Finally, radial containment of ions within a linear ion trap results in strong focusing along the centerline of the trap, in contrast to the 3-D trap in which fields tend to focus the trapped ions to a point. Line rather than point focusing properties may have tin influence on the relative susceptibilities to space charge phenomena.
Ions can be trapped within a linear ion trap and mass selectively ejected in a dimension perpendicular to the center axis of the trap, via radial excitation techniques. Exemplary devices for radial ejection trap ions in the radial dimension by an RF quadrupole field, and by static DC potentials at the ends of the rod structure. Many of the scan functions commonly used in conventional 3-D ion traps can also be applied to these linear 2-D ion traps. Upon ejection, ions emerge radially over the length of the quadrupole rod structure and can be detected using conventional means. Radial mass-selective ion ejection occurs when the RF voltage is ramped in the presence of a sufficiently intense auxiliary AC voltage. The auxiliary AC resonance-ejection voltage is applied radially and the ions emerge from the linear ion trap through slots cut in the quadrupole rods. Radial ejection requires that the RF field be of high quality over the entire length of the ion trap in order to preserve mass spectral resolution, since resolution depends on the fidelity of the secular frequency of the trapped ions. Thus, very high mechanical precision is required in fabrication of the quadrupole rods in order to maintain the same secular frequency over the length of the device.
There are several disadvantages of radial ejection of ions from a two-dimensional RF quadrupole. One disadvantage is that radial ejection expels ions through or between the quadrupole (or higher order multipole) rods. This forces the ions through regions of space for which these are significant RF field imperfections. The effect of these imperfections is to eject ions at points not predicted by the normal stability diagram. Radial ejection from a two-dimensional RF quadrupole has the further disadvantage of providing a poor match between the dimensions of the plug of ejected ions and conventional ion detectors. In a linear or curved rod structure, radially ejected ions will exit throughout the length of the device, i.e. with a rectangular cross-section of length corresponding to the rods themselves. Most conventional ion detectors have relatively small circular acceptance apertures (e.g. less than 2 cm2) that are not well-suited for elongated ion sources. Mass selective instability for radial ion ejection of ions from a two-dimensional RF quadrupole has additional problems. Ions ejected radially from such a device will exit with a diverging spacial profile with a characteristic solid angle. Same of the ejected ions will hit the rods and be lost. In addition, radially ejected ions will leave the trapping structure in opposite directions. Multiple ion detectors would be required to collect all the ions made unstable by similar techniques. Ions ejected away from the detector(s) or which encounter one of the electrodes are lost and therefore do not contribute to the measured ion signal. Therefore, only a small fraction of trapped ions would normally be collected, despite the very high storage ability of this device.
Mass-selective axial ejection (MSAE) of ions from linear quadrupole ion traps allows ions to be ejected axially, which can be a better special match for detectors. Most MSAE systems take advantage of RF fringing fields at the axial end of a quadrupole to convert radial ion excitation into axial ion ejection in a manner analogous to resolving RF-only mass spectrometers.
Trapped ions are given some degree of radial excitation via a resonance excitation process, and in the exit fringing-field, this radial excitation results in additional axial ion kinetic energy that can overcome an exit DC barrier. MSAE of ions from a linear quadrupole ion trap has been shown to add high-sensitivity and high-resolution capabilities to traditional triple quad mass spectrometers. Trapped, thermalized ions can be ejected axially in a mass-selective way by ramping the amplitude of the RF drive, to bring ions of increasingly higher m/z (mass to charge ratio) into resonance with a single-frequency dipolar auxiliary signal, applied between two opposing rods. In response to the auxiliary signal, ions gain radial amplitude until they are ejected axially or neutralized on the rods. In general, the radial excitation voltage is lower than that used to perform mass-selective radial ejection since the goal is provide a degree of radial excitation rather than radial ejection.
Several techniques have been proposed in the prior art for effecting axial ejection of ions from a linear ion trap. Exemplary systems for MSAE utilizing fringing fields is described in Hager, J. W., A New Linear Ion Trap Mass Spectrometer; Rapid Commun. Mass Spectrum, 2002; 16:512-526, and U.S. Pat. No. 6,177,688. The electric field responsible for MSAE of ions trapped in a linear quadrupole ion trap have been studied and characterized in the prior art. For example, such electric fields are discussed in detail in Londry, F. A.; Hager, J. W., Mass Selective Axial Ion Ejection from a Linear Quadrupole Ion Trap, J. Am. Soc. Mass. Spectrom. 2003, 14:1130-1147. In a conventional quadrupole ion trap utilizing MSAE, axial ejection occurs as a consequence of the trapped ions' radial motion, which is characterized by extrema that are phase-synchronous with the local RF potential. As a result, the net axial electric field experienced by ions in the fringe region, over one RF cycle, is positive. This axial field depends strongly on both the axial and radial ion coordinates. The superposition of a repulsive potential applied to an exit lens with the diminishing quadrupole potential in the fringing region near the end of a quadrupole rod array can give rise to an approximately conical surface on which the net axial force experienced by an ion, averaged over one RF cycle, is zero. This conical surface can be referred to as the cone of reflection because it divides the regions of ion reflection and ion ejection. Once, an ion penetrates this surface, it feels a strong net positive axial force and is accelerated toward the exit lens. As a consequence of the strong dependence of the axial field on radial displacement, trapped thermalized ions can be ejected axially from a linear ion trap in a mass-selective way when their radial amplitude is increased through a resonant response to an auxiliary signal.
The above mentioned MSAE ion trap systems are used in the ion path of a linear mass spectrometer. While this ion path may include a plurality of quadrupole sections, in general only the last quadrupole section is utilized as an ion trap, with initial quadrupole stages assisting in collimating the ion path in the axial direction. Ion injection is accomplished, in these examples, utilizing the fringing fields that occur at the radial and of the parallel rods that form the quadrupole of the ion trap. The quadrupole rods in the ion trap are substantially equal in length and parallel.